This invention relates to a projector for projecting an enlarged optical image on a screen.
As a device for displaying a large image, there has been known a projector which generates an optical image in accordance with a video signal by illumination light and projecting the enlarged optical image of illumination light on a screen by means of a projection lens.
Such a projector will be briefly described with reference to FIG. 8 showing its construction. Beams transmitted from an illuminating optical system 51 are split into beams in three wavelength regions of R (Red), G (Green) and B (Blue) by dichroic mirrors 52, 53. Specifically, the beams in the wavelength region of R reflected by the dichroic mirror 52 are reflected by a full-reflection mirror 54, and illuminate a liquid crystal panel 56 after passing a field lens 55. The beams in the wavelength regions of G-and B pass the dichroic mirror 52. The beams in the wavelength region of G are reflected by the dichroic mirror 53, and illuminate a liquid crystal panel 58 after passing a field lens 57. The beams in the wavelength region of B pass the dichroic mirror 53, and are introduced to a relay optical system constructed by two lenses 59, 60 and two full-reflection mirrors 61, 62. Thereafter, these beams illuminate a liquid crystal panel 64 after passing a field lens 63.
Optical images formed by the respective three liquid crystal panels 56, 58, 64 are combined by a dichroic prism 65. Specifically, the optical image of R formed by the liquid crystal panel 56 is propagated straight in the dichroic prism 65 as an incident light, is reflected at 90.degree. by a first dichroic mirror portion 65a after having been incident thereon at 45.degree., and emerges toward a projection lens 66. The optical image of B also is propagated straight in the dichroic prism 65 as an incident light, is reflected at 90.degree. by a second dichroic mirror portion 65b after having been incident thereon at 45.degree., and emerges toward the projection lens 66. The optical image of G formed by the liquid crystal panel 58 is propagated straight all through the dichroic mirror 65 as an incident light without being reflected by the first and second dichroic mirror portions 65a, 65b, and emerges toward the projection lens 66.
In this way, the optical images formed by the respective three liquid crystal panels 56, 58, 64 are combined by being projected toward the projection lens 66 located in the same direction while the axes of polarization of the optical images are coincided with an optic axis. The thus combined optical image is projected as an enlarged image on the screen via the projection lens 66.
However, if an optical integrator is used in the illuminating optical system 51 of the above prior art projector in order to make the illumination as bright as possible and to illuminate the image surfaces of the respective liquid crystal panels with a more uniform illuminance, the angle of incidence on each dichroic mirror varies to a greater degree than in the projector in which no optical integrator is used. This results in a larger degree of chromaticity nonuniformity of the image projected on the screen.
Specifically, if the optical integrator is used, an incidence angle of the beam on the dichroic mirror has a range of .+-.20.degree. with respect to a center value of 45.degree.. Even if the beams are incident on the dichroic mirror at an incidence angle of 25 to 65.degree., it does not cause any problem unless a cutoff value of the dichroic mirror for target wavelengths varies. However, in reality, such a variation is unavoidable. In FIG. 8, for example, in the case of the beams in the wavelength region of G, the chromaticity (Yxy value) of the green beams illuminating the liquid crystal panel 58 varies at a left point P1 and a right point P2 of the liquid crystal panel 58 (see FIG. 10) due to an incidence angle dependency of the dichroic mirror 53 (see FIG. 9). Thus, the image having its chromaticity varied by the projection lens 66 is projected on the screen as it is, disadvantageously causing a color unevenness.
In order to correct this color unevenness, it is necessary to provide, for example, a dichroic mirror for a color correction purpose at an incidence side of the dichroic prism 65.. In this case, the number of parts is increased.